1. Field of the Invention
The present invention relates to an optical head device which reproduces information recorded in a multilayered optical disk.
2. Description of the Related Art
In a situation in which an infrastructure capable of enjoying digital information of a dynamic image increasingly upgrades and expands such as satellite digital broadcasting or ground digital broadcasting, there is a demand for realization of a conveyable recording medium capable of recording a larger capacity of higher-definition digital information. To meet this demand, there are developed various types of next-generation high-density optical disks each having a recording capacity three or four times that of a presently spreading digital versatile disk (DVD). Among them, there is developed an HD DVD using a blue purple laser diode having a 405 nm wavelength band and an objective lens having an optimized substrate thickness of 0.6 mm and a numerical aperture of 0.65, from viewpoints of compatibility with the existing compact disk (CD) and DVD, ease of realizing a thin optical head device for a notebook-size personal computer, a low drive manufacturing cost, and a low disk manufacturing cost. In the HD DVD, development of a double-layer disk including double recording layers is advanced in parallel in the same manner as in the DVD for a purpose of further increasing a recording information amount.
The substrate thickness of the double-layer disk deviates from that of a single-layer disk, which is 0.6 mm. Therefore, a wave aberration is generated in a beam spot, and optical characteristics deteriorate. To suppress the wave aberration to an allowable value or less in the double-layer disk of the existing DVD, an interlayer thickness between two recording layers is defined as 55 μm±15 μm. When aberration standards similar to those of the DVD are applied to the HD-DVD, the interlayer thickness between two layers is reduced to about 25 μm. This is because the wave aberration generated with respect to an error of the substrate thickness is substantially proportional to the fourth power of the objective lens numerical aperture, and inversely proportional to a laser wavelength. Since the interlayer thickness of the HD DVD becomes smaller than that of the DVD in this manner, there is a remarkable influence of a phenomenon where undesired light reflected by a non-reproduction layer of two recording layers leaks to and falls on a photodetector, which is a so-called interlayer crosstalk. That is, since the interlayer thickness is small in the HD DVD, a beam diameter of the leak light from the non-reproduction layer on the face of the photodetector is smaller than that of the DVD. Therefore, a quantity of leak light increases. This is a cause for deterioration of a reproduction signal. Therefore, in addition to defining of the interlayer thickness, an interlayer crosstalk reducing measure is necessary in the HD DVD.
Jpn. Pat. Appln. KOKAI Publication No. 2003-323736 discloses an interlayer crosstalk reducing method due to a pinhole element with reference to, for example, FIG. 1. A position of the pinhole element is adjusted to pass signal light from a reproduction layer. Since the beam diameter of the leak light from the non-reproduction layer increases on the pinhole element, a large part of the leak light is interrupted by the pinhole element. That is, the signal light can pass through the pinhole element, but a large part of the leak light cannot pass through the pinhole element. As a result, the interlayer crosstalk is effectively reduced. The position of the pinhole element needs to be mechanically adjusted with good precision with respect to three axes, that is, an optical axis (z-axis) and two axes (x-axis and y-axis) crossing each other at right angles in a plane perpendicular to the optical axis. Therefore, the pinhole element is usable in an experimental level, but it is substantially difficult to mount the element on a product.
Jpn. Pat. Appln. KOKAI Publication No. 6-180851 discloses that a liquid crystal shutter whose aperture size and position are adjustable is disposed in a focal plane of a lens for condensing reflected light from an optical information recording medium or near the focal plane with reference to, for example, FIG. 1. This liquid crystal shutter is disposed for a purpose of applying to the photodetector a light beam having a diameter which is smaller than a diffraction limit diameter of the condensing lens, and the shutter is not used for the reduction of the interlayer crosstalk.